The invention relates to a single vision spectacle lens according to the preamble of the patent claim 1 or 2.
A spectacle lens on which the wording of the preamble of the patent claim 1 has been based is known from the Example 7 of EP 0 560 999 A1. This spectacle lens has a rotationally symmetrical aspherical surface and an atoroidal surface.
EP 0 560 999 A1 is mainly concerned with reducing the lens thickness and the image defects of spectacle lenses having astigmatic power. Howeverxe2x80x94as can be seen from FIGS. 21a and 21b of EP 0 560 999 A1xe2x80x94in the case of astigmatic defects as well as power deviations xe2x80x9cresidual defectsxe2x80x9d of at least 0.1 dpt remain on both principal meridians.
From DE 195 40 186 a spectacle lens with two aspherical surfaces is known, of which at least one is configured to be rotationally symmetrical. The object of the mentioned patent application is also to reduce the thickness and the image defects of a spectacle lens. However, the specification of this patent application expressly states that the image defects increase along the radial direction with the distance r from the vertex. With this, the image defects may attain values greater than 0.1 dpt at a 20 mm distance. Furthermore, there is no preferred direction along which the image defects are specifically corrected.
Reference is expressly made to both of the two above mentioned publications concerning an explanation of all details not described here more explicitly.
In accordance with the present invention it has been realized that it is possible to design a single power spectacle lens so that neither an astigmatic defect nor a deviation of power will occur along at least one line at a viewing angle of up to 40 degrees.
Thus it is the underlying object of the present invention to specify a spectacle lens which is well optimized as far as possible with respect to the astigmatic defect and/or the power deviation, wherein aspects of appearance, such as the curvature of the front surface, have not been over-looked but not been given primary consideration.
In particular, it is intended to correct astigmatic defects and refraction defects advantageously as far as possible with respect to all directions of viewing. In addition, the faces or surfaces of the spectacle lens are to be optimized to reduce the mentioned defects. At the same time nevertheless, any additional outlay is to be kept small.
An achievement of this object in accordance with the present invention is set out in the patent claims 1 and 2. Further developments of the invention are the subject matter of the dependent claims.
In accordance with the present invention it has been realized that a correction of both image defects, i.e. those of astigmatism as well as those of refraction, is not possible for all directions of viewing. This also applies when two aspherical surfaces are used. It is possible to correct either only the astigmatism or only the refractive defect for all angles of viewing. With a second aspherical surface only the properties relating to appearance, such as the thickness or the curvature of the front surface, may be further improved (see DE 195 40 186 A1).
It is therefore the basis of the invention that even if the image defects cannot be corrected for all angles of viewing, this can be done, as distinct from prior art, for one or more preferred directions.
In general, such a preferred direction is the horizontal and/or the vertical direction. However, it may also be any other desired direction according to the intended use (e.g. by golf players). For this, however, the image defects for the other viewing directions (up to 40 degrees of deviation) must not exceed any values which strongly impair the direct vision. These are values of, for example, 1.0 dpt.
In order to achieve this, at least one atoroidal surface is needed. The second surface may then be chosen as desired, but is preferably a rotationally symmetrical surface.
The atoroidal surface may be a surface having two or one or no planes of symmetry. If the preferred directions for the correction and the cylinder axis (axial orientation) of astigmatism do not coincidexe2x80x94which is relatively frequently the casexe2x80x94, then the atoroidal surface preferably no longer has any plane of symmetry.
Even with a purely spherical prescription, at least one atoroidal surface is needed, as distinct from prior art. Then either an atoroidal surface is used for which the vertex curvatures are the samexe2x80x94i.e. surface astigmatism occurs only outside the center or vertexxe2x80x94, which is to be preferred, or thexe2x80x94actually undesiredxe2x80x94astigmatism at the vertex must be compensated with a second toroidal or atoroidal surface.
According to the invention it is of advantage for the number of lines specified in claim 1 to be greater than 1. The surfaces of the spectacle lenses may be additionally adapted according to the optical requirements. Also the shape as well as the position of the line or the lines can be advantageously conformed to the optical requirements.